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1 i <br /> Ground Water Sampling <br /> 1 <br /> • volatilization and/or sensitivity to oxidation/reduction reactions The well <br />' should be retested for dissolved oxygen and turbidity after sampling as a <br /> measure of purging efficiency and as a check on the stability of the water <br /> samples over time <br />' O A well purged to dryness should be sampled as soon as a sufficient <br /> volume of ground water has entered the well to enable the collection of <br /> the necessary ground-water samples <br /> 1 O The RP should ensure that purging does not cause formation water to <br /> cascade down the sides of the well screen(this may occur when the water <br />' level in the well is lowered into or below the screened interval) At no <br /> time should a well be purged to dryness if recharge causes formation <br /> water to cascade down the sides of the screen, as this may cause an <br />' accelerated loss of volatile constituents, resulting in a sample not <br /> representative of actual ground-water quality This problem should be <br /> anticipated,water should be purged from the well at a rate that does not <br />' cause recharge water to be excessively agitated Laboratory experiments <br /> have shown that unless cascading is prevented, up to 70 percent of the <br /> volatiles present could be lost before sampling <br />' o If the purged water is contaminated,based on prior test results,the water <br /> should be stored in appropriate containers until analytical results are <br /> available, at which time proper arrangements for disposal or treatment <br /> should be made (i e , contaminated purge water may be a hazardous <br /> lie waste) Purge water from new wells, for which there are no prior <br /> chemical data, should be containerized and assumed potentially <br /> contaminated until sample analytical results prove otherwise <br /> 224 Frequency of Sampling <br /> Sampling frequency, in nearly all cases, should be based on the hydrogeology of <br /> the site There is no maximum sampling frequency set by Cal/EPA Ground- <br /> water analytical results should be reviewed periodically, and sampling frequency <br />' modified according to data needs, historical water quality trends and regulatory <br /> goals Cal/EPA recommends a minimum of quarterly sampling for at least the <br /> first year of monitoring To track potential seasonal changes in concentration, at <br />' least two sampling rounds should roughly coincide with maximum and minimum <br /> water table or potentiometric surface elevations EPA's guidance document <br /> "Statistical Analysis of Ground-Water Monitoring Data at RCRA Facilities, <br />' Interim Firs.Guidance(EPA/530-SW-89-026, DUMS PB-89-151-047, U S EPA <br /> 1989a) suggests a method for choosing a sampling interval that will reflect <br /> site-specific hydrogeologic conditions The method uses the Darcy equation to <br />' determine the horizontal component of the average linear velocity of ground water <br /> for confined, semi-confined, and unconfined aquifers This value is used to <br /> determine a sampling interval that will yield an independent sample of ground <br /> water in diffuse flow regimes <br /> Recent research performed to the area of ground-water sampling frequency <br /> (Barcelona et al , 1989) indicates that ground-water monitoring data should be <br /> 8 <br />